Dental caries is an endemic disease in humans. The primary pathogen of caries on coronal surfaces of teeth is thought to be Streptococcus mutans although other bacteria are implicated in the progression or extension of lesions. Accordingly, measures directed specifically against S. mutans have been included in the many therapies and preventative treatments used against caries. Among the most recent of these have been attempts at the selective replacement of S. mutans in dental plaque by avirulent oral streptococci occupying the same niche on tooth surfaces as S. mutans. Ideally, one would hope to implant a strain or mutant "more fit" for that niche than the originial occupant, namely S. mutans. Thus, it becomes of importance and interest to understand ecological competition among oral streptococci. In this application it is proposed to investigate the roles of uptake systems for glucose, sucrose, and arginine in competition between pairs of oral streptococci grown in mixed culture in a nutrient-limited chemostat. The organisms to be studied are S. mutans Ny344, S. sanguis P4A7, and S. milleri B448. Nutrient-limited chemostats have been chosen as a model for nutrient-limited microbial growth seen in natural ecosystems. Each organism will be grown in a chemostat limited for glucose, sucrose, or arginine, and Km's for uptake of these limiting nutrients will be determined with cells taken from the corresponding chemostats. Then, mixed cultures starting with equal numbers of S. mutans and either S. sanguis or S. milleri will be grown in chemostats limited for glucose, sucrose, or arginine, and the outcome of competition between the two organisms determined by measuring the numbers of the two organisms in the culture as the chemostat comes to a steady state equilibrium. It is expected that the organism having the lower Km for the limiting nutrient will predominate in the mixed culture. This project provides the opportunity to obtain physiological explanations for ecological observations.